I agree, you have to concede his first outing went about as well as could be expected, no doubt, and I think he probably is a very remarkable pitcher. But like you said, time, and scouting reports, and repeated viewings, particularly in the unfriendly pitching environment of the AL, can change the way a pitcher performs over the course of a season, and ultimately, a career.
Plus, I don't think he has pitched as long of a season as he'll have to pitch this year. I'm not sure how long Japan's season is but I've heard a few guys wondering if he'll simply burn out by the end of the season.